How to measure LED lamp life

There has been a lot of discussion about the issue of measuring the lifetime of solid-state lighting. Industry watchers say standards in this area are probably still in the future. Nevertheless, there was progress on the topic at a recent panel convened during the Dept. of Energy's (DoE) sixth annual SSL Market Introduction Workshop in Seattle. According to the DoE's Lighting Facts website, the consensus there seemed to be that the industry better understands SSL lifetime and reliability and that it goes far beyond lumen maintenance of the LEDs.

A recent report produced by a working group under the auspices of DoE and the Next Generation Lighting Industry Alliance (NGLIA) reflects the most current thinking on LED lamp lifetime. Called LED Luminaire Lifetime: Recommendations for Testing and Reporting, the report says we really don't know the true reliability and lifetime of light-emitting diode (LED) lighting systems. Lumen maintenance values of LED devices are widely used as a proxy for the lifetime of an LED lighting system, it says, which is misleading: Lumen maintenance is just one component of luminaire reliability.

In fact, LED lumen depreciation quite often doesn't determine the lifetime of a well-made luminaire. The report further says many manufacturers estimate luminaire lifetime using LED lumen maintenance figures, which can give misleading results.

This practice can be particularly bad for fixture manufacturers because it impacts whether or not the fixture will last as long as the manufacturer's warranty. "The statement '100,000 hours of LED luminaire lifetime' is gradually giving way to the realization that there is little consistency, very little published data, and few hard facts around so-called luminaire lifetime numbers," says the report. But the situation is better at the LED package level, it says, where reputable manufacturers have thousands of hours of data under varying conditions.

All in all, there is still no accepted protocol for measuring and characterizing LED fixture lifetime. One reason is that it can be costly to make such measurements due to the large number of product samples such tests require. "But as we learn more about the behavior of these LED 'systems,' it becomes evident that the life of a fixture may be considerably shorter than what is indicated by nominal light depreciation, albeit still generally longer than many incumbent lighting solutions," the report says.

The full text of the report is free and available here:

http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/led_luminaire-lifetime-guide_june2011.pdf